Many golf balls are commercially selling, but they are typically classified into solid golf balls such as two-piece golf ball, three-piece golf ball and the like, and thread wound golf balls. The solid golf ball consists of a solid core of molded rubber material and a cover of thermoplastic resin (e.g. ionomer resin) covering on the solid core. The thread wound golf ball consists of a solid or liquid center, a thread wound layer formed on the center and a cover of ionomer resin or balata etc. having a thickness of 1 to 2 mm covering on the thread wound layer.
The thread wound golf ball, when compared with the solid golf ball, has better shot feel at the time of hitting and better controllability at approach shot. The thread wound golf ball is generally approved of or employed by high level golfers, especially professional golfers, who regard the characteristics as most important. On the other hand, the thread wound golf ball is inferior in flight distance to the solid golf ball. Therefore, it is required to provide the thread wound golf ball having sufficient flight distance, while maintaining the advantage of having good shot feel and excellent controllability.
In the thread wound golf balls, there are two types, such as one comprising a solid center formed from integrally molded rubber material and the other comprising a liquid center composed of a hollow rubber sphere and liquid encapsulated in the sphere. The thread wound golf ball consists of the solid or liquid center, a thread wound layer formed on the center and a cover of ionomer resin or balata etc. covering on the thread wound layer. The conventional solid center of rubber material, which is formed from vulcanized butadiene rubber, has very high hardness and small compression strain. The thread wound golf ball comprising the solid center has the disadvantage of having short flight distance, because the spin amount is large and the launch angle is small. The hit golf ball curves too much to control easily, sometimes.
On the other hand, the liquid center has large compression strain. The thread wound golf ball comprising the liquid center, when compared with the thread wound golf ball comprising the solid center, has the advantage of having long flight distance, because the spin amount is small and the launch angle is large. In the case of the liquid center, the production process is more complicated than that of the solid center. If the golf ball is cut by a cutter and the like, the liquid in the liquid center is scattered around. If the liquid comes into eyes, one would unfortunately lose one's eyesight.
The present inventors have found that the spin amount is restrained low and the launch angle is high to extend the flight distance together when hit by a wood club and an iron club, while maintaining proper initial velocity, by employing a solid rubber having a crosslinked structure containing an oily substance as a center of thread wound golf ball (Japanese Patent Kokai Publication No. 337217/1993). However, the oily substance of the center causes bleeding with time to penetrate into the thread rubber layer, which results in deterioration of rebound characteristics. Therefore there was a problem that the performances of the resulting golf ball could not be maintained.
The present inventors have found that the performances of the resulting golf ball can be maintained, by coating around the solid rubber center containing an oily substance with an oily-resistant substance to prevent the bleeding of the oily substance (Japanese Patent No. 2664857, Japanese Patent Kokai Publication No. 173504/1997). However, since an oily-resistant rubber or ionomer resin having high hardness are employed as an oily-resistant substance for coating around the solid rubber center containing an oily substance, it is required to improve the rebound characteristics and shot feel.